1. Field of the Invention
The present invention generally relates to gravity operated switching circuits for power supplies for electronic equipment and, more particularly, to a switching circuit having particular application in end of train signalling equipment which automatically turns the signalling equipment off with a predetermined time delay after the equipment is removed from use and laid on its side for storage.
2. BACKGROUND OF THE INVENTION
It is known in the prior art to provide electrical and electronic devices with a gravity operated switch so that, if the device is tipped over or intentionally laid on its side, the power to the device is turned off. The purpose of turning off the power may be to conserve power, as in battery powered devices, or as a safety precaution. An example of the former is shown in U.S. Pat. No. 2,492,837 to Briggs which discloses a portable signal light, powered by a battery, which includes a mercury switch that is open when the light lays on its side. The switch is closed only when the light is in a substantially upright position. As a safety precaution, such switches may be incorporated into electrical appliances to prevent accidental fires if tipped over while unattended.
End of train signalling and monitoring equipment is becoming more widely used as a replacement for the traditional caboose. The information monitored, which typically includes the air pressure of the air brake line, is communicated by a battery powered telemetry transmitter to the crew in the locomotive. In addition, a marker light is required at the end of the train, and this too is battery powered. The end of train signalling and monitoring equipment is typically assembled into a compact unit which is designed to be mounted on the trailing coupler of the last car of the train.
Modern end of train equipment includes a microprocessor with a stored program designed to control the monitoring and signalling functions of the equipment. Since the equipment is battery powered and it is important to conserve the power of the battery when the equipment is temporarily not in use, it is known to provide this equipment with a gravity operated switch, such as a mercury switch, so that when the equipment is removed from the coupler and laid on its side, many of the equipment functions are turned off. This operation is controlled by the microprocessor which may be programmed to provide different time constants for turning the equipment off and, when remounted on the coupler, on again. Basically, the functions which are controlled in this manner, that is turned off, are the flashing of the marker light and the the transmission of data by the telemetry transmitter. In order for this system to function, it is necessary for the microprocessor and all the related circuitry to be continuously powered, and this in turn produces a continuous drain on the battery.